1. Field of the Invention
This invention relates to an automatic photometer for metering light, such as transmitted light, reflected light, fluorescence or phosphorescence, from a sample to be measured.
2. Description of the Prior Art
A prior art photometer has employed an optical system as shown in FIG. 1 of the accompanying drawings. In such optical system, a light source 1 illuminates a sample 2 whose image is focused through an objective 3 onto a stop plate 4. Upon illumination from the light source 1, the sample 2 also provides a background having a certain intensity of brightness in itself. Further, the sample has a portion 2a to be measured as bright as the background itself, a portion 2b to be measured brighter than the portion 2a, and a portion 2c to be measured still brighter than the portion 2b (see FIG. 2). A light receiving element 5 is arranged to receive only that part of the light which has been passed through an opening 4' in the stop plate 4.
With such arrangement, the light from the sample portion 2b can be fully received by the light receiving element 5 to measure the entire portion 2b when the image 2'b of such portion is wholly contained within the opening 4' of the stop plate 4 in the manner as shown in FIG. 3a. In the position as shown in FIG. 3b or 3c, however, the image 2'b or 2'c of the sample portion 2b or 2c is not wholly contained within the opening 4' of the stop plate with the result that light from only a part of the image 2'b or 2'c is received for photometry by the light receiving element, thus providing an erroneous photometric indication. In other words, accurate photometry cannot be achieved unless the operator confirms at each test whether the image of the sample is completely aligned with the opening 4' of the stop plate. Thus, the prior art photometer could not accomplish automatic photometry.